1. Field of the Invention
This invention relates to a cutting tool stop-control apparatus for controlling the stopping of a cutting tool when executing a hole drilling operation by a machining center or milling machine, and particularly when executing a back-boring operation.
2. Description of the Related Art
In order to machine a workpiece by an ordinary CNC machine tool, a program for controlling the movement of a tool in dependence upon the contents of machining is created and the machine tool is run in an NC mode based on the program. When an abnormal condition such as tool breakage occurs on the side of the machine tool, the numerical control apparatus is capable of interrupting cycle operation, which is based upon the cycle program in the process of being run. On the side of the machine tool, the cutting tool is brought to an emergency stop, after which the tool is changed. In order to resume the prescribed operation applied to the workpiece, it is required that the tool be withdrawn to the cycle starting point.
With a numerical control apparatus of this kind, generally a method is adopted in which the tool is returned to the cycle starting point while the path of withdrawal is read out as the reverse of the same path traversed. For example, if operation is suspended when a program is being executed in a machining mode having a complicated path, as in the case of a back-boring cycle, the operation for withdrawing the tool to the cycle starting point is a major undertaking. In the prior art, therefore, operation from this time onward is switched over to manual to make the tool change. This is disadvantageous in that the efficiency of operation declines.